Bazel 0.4 doesn't build on Darwin

[abbradar]

Issue description

We didn't have it working before TensorFlow started to build from source, but we need it now to fix TensorFlow on Darwin. Unfortunately I don't have a Mac so I can't tackle this problem efficiently. Current error on Hydra:

🍃  Building Bazel from scratch......./usr/bin/xcrun clang -fobjc-arc -framework CoreServices -framework Foundation -o /private/tmp/nix-build-bazel-0.4.5.drv-0/bazel_6mTHoLID/archive/_embedded_binaries/xcode-locator tools/osx/xcode_locator.m
xcode-select: error: no developer tools were found at '/Applications/Xcode.app', and no install could be requested (perhaps no UI is present), please install manually from 'developer.apple.com'.
/nix/store/ncvjpvcpsixl8vx2g8nm71m25xyhx67w-stdenv-darwin/setup: line 1055: ./output/bazel: No such file or directory

How should this be fixed?

Worst case I'll retain TensorFlow binary packages specially for Darwin.

Steps to reproduce

1. nix-build -A bazel_0_4

Technical details

• Nixpkgs version: (run nix-instantiate --eval '<nixpkgs>' -A lib.nixpkgsVersion) 9a84b52

[LnL7]

replacing /usr/bin/xcrun clang with clang or cc usually fixes these kind of issues.

[abbradar]

What does /usr/bin/xcrun --sdk ${sdkString} --show-sdk-path return?

[copumpkin]

If it really insists on there being an SDK path, we do actually have a fake xcrun that returns sensible data for our pure toolchain. Just include xcbuild in your buildInputs.

[mboes]

Close? cc @Profpatsch @uri-canva

[Profpatsch]

Let’s see what ofborg says in #42735 (comment)

[uri-canva]

I'm not using bazel 0.4 on darwin so I haven't looked into it, sorry.

[Profpatsch]

Hm, I think we can close, since bazel is not yet buildable on Darwin and the newest version is 0.15.1. We are working on getting it to build there, though.